During the operation of electrical or electronic equipment, electric and magnetic fields are generated which can interfere with the operation of other electrical or electronic devices. This phenomenon is termed Electro-Magnetic Interference (EMI). In the past, this has not been a significant problem for two reasons: first, there were many less electrical devices than today, and, more importantly, most of these devices were constructed of metal enclosures which effectively shielded this interference.
Today, however, the use of electrical and electronic equipment is growing at a substantial rate, and equipment enclosures which have previously been constructed of metal or other effective shielding materials have now given way to various plastics and similar materials that cannot shield EMI. Plastics have gained acceptance due to their light weight, high strength to weight ratio, ease of fabrication, corrosion resistance, and consolidation of parts, all of which contribute to the consumer appeal and lower cost of these electronic devices.
EMI is a wave which travels in a similar manner to light. When encountering construction materials, these waves can be reflected, absorbed, or transmitted depending upon the electrical properties (particularly the electrical conductivity) of the material. As mentioned above, metals are effective for EMI shielding because they have high electrical conductivities and absorb the EMI waves. On the other hand, plastics are insulators, and they allow these EMI waves to pass unimpeded through their structure.
The EMI waves that are generated by electrical or electronic equipment are comprised of two components. The electric field (E) component can penetrate any plastic or non-metallic material, but are absorbed by grounded metals or other materials having high electrical conductivity. The magnetic field (H) can penetrate all plastics and metals, but loses its effectiveness fairly rapidly as a function of distance from the source. In contrast, the electric fields are capable of creating interference over much greater distances.
The two components of electromagnetic waves propagate at right angles to the plane containing them. The relative magnitude of these fields are a function of distance between the generating source and receiver. The ratio of E to H is defined as the wave impedence, Zw. A low Zw occurs when the source contains a low voltage and high current. This situation is called a current source, magnetic source, or low impedance source, and a typical example is a transformer. Conversely, a high voltage with small current flow results in a high impedance, and the resultant field is primarily electrical.
When an EMI wave encounters a material, the wave will reflect if its impedance and that of the material substantially differ, and minimal energy will be transmitted across the boundary. The principle behind EMI shielding is to use a shielding material of low impedance, such as highly conductive metals, to reflect EMI waves. Magnetic waves have low impedance similar to metals, so that energy is transferred through the metal with minimal reflection. Consequently, the magnetic component of EMI waves can be difficult to shield, although, with distance, their effect significantly decreases. Since the electric component becomes the greater source of problems at farther distances, low impedance materials are used.
EMI waves pass through free space at 3.times.10.sup.8 M/sec and through other non-conductors such as plastic at essentially the same speed. When one places a conductive barrier to shield EMI, those waves strike the barrier and some of the energy is reflected back similar to light reflected from a mirror. Energy entering the conductive shield generates eddy currents in the conductor, further attentuating the wave's strength.
At higher frequencies, reflection decreases and absorption increases, and as a result, the reflected wave and the absorbed wave increase in materials of higher conductivity. As the reflected wave is independent of thickness, the relative thickness of the shielding has little effect on the reflected wave while the absorbed wave is strongly dependent on the shield thickness and thus its related attenuation level increases with the thickness of the barrier form.
The effectiveness of a shield is determined by measuring the separate field strength of a unit with and without a shield. The effectiveness of a shield is measured in decibels (db) as the ratio of field intensities before and after shielding. Measurements of Shielding Effectiveness (S.E.) can be of the electrical field (E), magnetic field (H), or power (P) as defined by one of the following equations: EQU S.E.=20 log E.sub.b /E.sub.a (Volts/meter) EQU S.E.=20 log H.sub.b /H.sub.a (Amps/meter) EQU S.E.=10 log P.sub.b /P.sub.a (Watts/square meter)
Where:
E.sub.b =the electric field strength before the shield is installed PA1 E.sub.a =the electric field strength after the shield is installed PA1 H.sub.b =the magnetic field strength before the shield is installed PA1 H.sub.a =the magnetic field strength after the shield is installed PA1 P.sub.b =the power of the field before the shield is installed PA1 P.sub.a =the power of the field after the shield is installed
Each 10 dB increment of attentuation or dissipation equates to a ten fold improvement in shielding effectiveness. A 10 dB attenuation means the signal strength reaching the sensor was reduced to 10% of source energy, 20 dB to 1% of source, etc. Typical values for shielding quality are shown below in Table I.:
TABLE 1 ______________________________________ SHIELD QUALITY dB attenuation Comments ______________________________________ 0-10 dB Very little shielding 10-30 dB Minimal shielding 30-60 dB Average 60-90 dB Above average 90-120 dB Maximum to beyond existing state of art. ______________________________________
It is estimated that an attenuation of at least 45 dB is required to provide adequate shielding for the electric or electronic equipment that is currently available today.
The degree of shielding effectiveness is not a constant for each shielding material, but is affected by the frequency of the incoming signal.
The shielding effectiveness is also determined by:
(a) The electrical conductivity of the shield.
(b) The thickness, uniformity, and smoothness of the shield.
(c) The physical properties of the shield.
(d) The frequency and impedance of the impinging field.
(e) The magnetic permeability of the material.
The electrical resistivity of the shielding material, measured in units of ohms/square (ohms/square is a non-dimensional measurement and does not refer to square inches or square feet) varies with the different materials used, and with the thickness of the material.
The frequency spectrum affected by EMI has traditionally been defined as the radio frequency range of 10 KiloHertz to 100 Gigahertz. As mentioned above, metals provide excellent shielding properties while plastics do not, but plastics are preferred for construction of electrical device enclosures for the reasons given previously. Therefore, the prior art has attempted a number of different methods for coating plastics with metals. Each type of shielding material has its own degree of effectiveness. The needs of the application dictate, for the most part, which type metal coating is needed to provide the proper protection. A description of the more successful systems follows.